Iodothyronine Interactions with the System L1 Amino Acid Exchanger in 3T3-L1 Adipocytes

Abstract

Thyroid hormones enter isolated white adipocytes largely by a System L1-type amino acid transporteren routeto exerting genomic actions. Differentiated 3T3-L1 mouse adipocytes in culture express mRNA for LAT1 (the catalytic subunit of high-affinity System L1). L-[125I]-T3uptake into 3T3-L1 adipocytes included a substantial saturable component inhibited by leucine. L-[3H]phenylalanine uptake into 3T3-L1 cells was saturable (Kmof 31 μM), competitively inhibited by T3(Kiof 1.2 μM) and blocked by leucine, BCH, and rT3as expected for substrate interactions of System L1. Efflux of preloaded L-[3H]phenylalanine from 3T3-L1 adipocytes wastransstimulated by external leucine, demonstrating the obligatory exchange mechanism of System L1 transport. T3(10 μM) did not significantlytransstimulate L-[3H]phenylalanine efflux, but did competitively inhibit thetransstimulatory effect of 10μM leucine. The results highlight strong competitive interactions between iodothyronines (T3, rT3) and amino acids for transport by System L1 in adipocytes, which may impact cellular iodothyronine exchanges during altered states of protein nutrition.